Intermediate Representation -> JSON without using old version

I'm planning on some [big changes to iOS codegen](https://github.com/apollographql/apollo-ios/issues/682) and I think from talking to @martijnwalraven, there's one major issue that can be solved on the Tooling end.

Right now, Android is generating code using the 1.x IR with [`target=json`](https://github.com/apollographql/apollo-android/blob/master/apollo-gradle-plugin/src/main/java/com/apollographql/apollo/gradle/CodegenGenerationTaskCommandArgsBuilder.java#L96) in order to generate code. From talking to Martijn and looking at the Gradle plugin, it seems like this is using an older version of the IR to generate the JSON that Android uses to generate the code.

Can we please get a `json` target which takes into account all the new work being done in v2.x of the Apollo CLI? It would be great because it'd allow flexibility for code generation in multiple languages.

The issue points to apollo-gradle-plugin/src/main/java/com/apollographql/apollo/gradle/CodegenGenerationTaskCommandArgsBuilder.java and the Apollo CLI's target=json path; begin by comparing that output with the v2.x IR work referenced in the issue. Done means a json target emits the current v2.x IR without relying on the old 1.x format and remains usable for Android and other code generators.

Written by the indexing model from the issue text.